Tuesday, January 9, 2018

A potentially significant winter storm is expected to impact the Ohio Valley and Northeast in the January 12-14 period, with all modes of wintry precipitation possible.

ARL

As of this morning, a strong upper level low was located offshore California, moving into the Southwest. As discussed in a previous post about the January 10-12 Winter Storm, this upper level low will actually be torn into two pieces by January 11th, as a piece of energy currently skirting the U.S. / Canada border near the Pacific Northwest will help a portion of the upper level low to become negatively-tilted and create the winter storm over the 10th through the 12th. This process is shown below.

ARL

As this piece of the low is taken northward and creates that winter storm, however, a deep positively-tilted trough is left behind in the southern Plains. This will be the energy that creates the January 12-14 potentially significant winter storm, expected to primarily affect the Ohio Valley and Northeast with wintry precipitation.

ARL

By Friday evening on January 12th, the aforementioned energy in the southern Plains is forecasted to move east into the Southeast and strengthen, to the point that it becomes negatively-tilted (shown in the northwest-to-southeast orientation of the vorticity maxima), indicating a mature storm system at peak / near-peak strength.

Tropical Tidbits

A few hours before that image is valid, the surface low will have already formed and is projected to be near eastern Tennessee by Friday afternoon. The swath of warm air that the January 10-12 Winter Storm had to deal with will again be present in the Northeast, allowing this event to begin as rain for the entirety of the Northeast and portions of the Ohio Valley. Northern Ohio and portions of Indiana may be able to retain temperature profiles cool enough to support wintry precipitation for nearly the entire event, but I have a feeling that will be more of a "nowcasting" determination.

Tropical Tidbits

By Saturday morning, as the trough continues to be at its peak intensity, the surface low is projected to be moving northeast and strengthening below the 1000-millibar level. As such, the deformation zone looks to be well-developed and producing heavy wintry precipitation from Kentucky through Ohio, much of New York, and slivers of Pennsylvania, Vermont and New Hampshire. As the graphic shows, a region of significant sleet and/or freezing rain is being outlined in eastern Ohio, northwest Pennsylvania and western New York at this time. I personally see freezing rain and, to a lesser degree, sleet as precipitation types that can only be accurately predicted immediately prior (i.e. 24 hours or less) to the event, simply because more factors are involved in getting freezing rain relative to the factors involved in producing snow or rain. So, while I will refrain from calling for a significant ice event from this storm, I certainly recognize the possibility of such an outcome, and model guidance on ice accretion will be analyzed further shortly.

Tropical Tidbits

By Saturday morning, the surface low is forecast to have strengthened further, with the deformation zone now in place from the northern Ohio Valley along the Great Lakes into extreme southeast Canada. Again, the GFS here anticipates a not-insignificant region of freezing rain and/or sleet, particularly from New York through northern Vermont and New Hampshire, as well as a good section of Maine.

In terms of accumulations, everything but the kitchen sink looks to fall from this storm.

Pivotal Weather

The heaviest accumulating snow is expected to fall from Kentucky into Ohio, with the highest totals projected in southern Canada along the Great Lakes. As is generally the case with dynamic winter storm forecasts more than 48 hours out, snow accumulation forecasts are rife with caveats. While it is wise not to see these numbers as written in stone, it is also wise to believe that this storm has the potential to produce significant snow totals in excess of 12". Exact amounts and the location of relative maxima in amounts will change as we get closer to the event, but it is quite apparent that this storm has the ability to lay down a swath of over 12" of snow across a sizable section of the Ohio Valley.

Pivotal Weather

I show this image of forecasted freezing rain accumulations as a word of caution more than anything. First, notice how jagged and strange the placement of these accumulations are, almost like a contorted snake. This is a symptom of the GFS being unable to resolve the environment to a fine enough degree - whereas the NAM model has far higher resolution (that's why NAM hi-res graphics look so pretty), the GFS and other global models have lower resolution, simply because they forecast for the entire world, as opposed to the NAM, which forecasts for the United States. If the GFS model was made to create global forecasts at the NAM's resolution out to 384 hours, the forecast run could take hours to complete! So, let's take a look now at the NAM's forecast for freezing rain accumulations.
Be warned, the NAM does not yet have the full timeframe of this storm in its sights yet, so ice accumulations will seem far lower than they may actually turn out to be.

Pivotal Weather

Now that's a lot easier on the eyes, isn't it? The NAM's ice accumulation forecast has a swath of freezing rain that looks far smoother, far less jagged and contorted than the GFS' image. Again, however, I post this partially as a word of caution. The NAM is notorious during the winter season for amplifying wintry precipitation accumulations beyond what they end up being. For example, in previous seasons it hasn't been uncommon for the NAM to project 8" of snow in one area while the GFS and ECMWF support up to 4" at best. This applies to freezing rain as well, and I believe that while the NAM is almost certainly over-doing ice accumulations here, it gives a good idea of where freezing rain *could* accumulate when this storm begins. Areas from northern Mississippi and eastern Arkansas through western Tennessee, western Kentucky, Indiana and Ohio could all be at risk for some ice from this storm. I'm a bit skeptical of the projected freezing rain in Louisiana, but with a storm system this dynamic it certainly bears watching.

To Summarize:

- A potentially significant winter storm is expected to impact the Ohio Valley and Northeast in the January 12-14 timeframe.
- While these regions will initially experience rain, a heavy snow event is expected for portions of Kentucky, Indiana, Ohio, and Canadian areas near the Great Lakes.
- Given the dynamic nature of this storm and the body of warm air affecting a region that only recently escaped from a "deep freeze", accumulating freezing rain is possible, particularly in a zone from western Tennessee/Kentucky and eastern Arkansas into Indiana, Ohio, and northern portions of Pennsylvania, New York, Vermont, New Hampshire and much, of Maine.
- While amounts will be refined in coming model runs, snow accumulations in excess of 12" are possible, particularly in Ohio and southern Canada.
- While amounts will likely be determined only shortly prior to the event, ice accumulations in excess of 0.25" are possible, particularly in Ohio, northern Pennsylvania, New York, Vermont, New Hampshire, and Maine.

A winter storm is expected to impact the Northern Plains over the January 10-12 period.

ARL

As of this morning, a strong upper level low was located offshore California, moving into the Southwest. In addition, a piece of energy was located moving onshore just north of the Pacific Northwest. These two features will combine to create an accumulating snow event for portions of the central U.S. in the next couple of days.

ARL

By Thursday morning, this upper level low is forecast to have moved into the Central U.S. and separated into two primary parts - the majority of the energy is projected to take the form of a positively-tilted trough, bottoming out in western Texas, while a smaller part of the low will break off and ride the ridge over the Ohio Valley to become negatively-tilted. At the same time, that small piece of energy we currently see moving along the U.S. / Canada border near the Pacific Northwest will help to pull that smaller piece of energy northward and help give it that negative tilt to create the winter storm for the Plains.

Tropical Tidbits

Also on Thursday morning, the winter storm is projected to be ongoing for the Plains, with a surface low located along the Iowa / Wisconsin border producing a band of accumulating snow from Kansas to Minnesota and the northern portions of Wisconsin. The GFS model portrays a sliver of precipitation to fall as sleet or perhaps freezing rain, but due to how conditions must be "just right" for freezing rain to occur, and to some extent also for sleet, the location and intensity of either freezing rain or sleet will likely only be accurately known just before the event begins. Rain is expected to fall and chip away at any enduring snowpack in Iowa, Illinois, Michigan, southern Wisconsin and parts of Missouri.

Pivotal Weather

Snow accumulations are expected to be maximized along a bad from central Kansas through eastern Nebraska, northwestern Iowa and southern Minnesota, with a stripe of particularly-heavier accumulations forecasted from southern Minnesota through northern Wisconsin and the upper peninsula of Michigan. Light snowfall is possible in the wake of the storm system as colder air filters in behind the low, resulting in the 1-2" accumulations depicted across southern Wisconsin, northern Illinois and parts of Iowa and Missouri.

The snowfall in the primary axis of heavy snow is expected to be of the heavier and wetter kind, as opposed to the light & fluffy snowfall seen from Alberta Clippers. As such, if you reside in an area expecting accumulating snowfall from this storm, be sure to take frequent breaks if you decide to shovel, and to not exert yourself when doing so. Snowblowers are often the best idea for moving substantial accumulations of this kind of heavy, wet snow.

To Summarize:

- A winter storm is expected to affect portions of the Central U.S. between January 10th and January 12th.
- Accumulations of 6-9" are expected in a band extending from central Kansas through eastern Nebraska, western Iowa, southern Minnesota, northern Wisconsin and the upper peninsula of Michigan.
- This snow is expected to be more akin to heavy, wet snowfall than lighter, fluffier snowfall. As such, take precautions when shoveling snow, and use a snowblower or plow if possible.

I'll begin with a topic that I've discussed a few times on this blog since November 2017: that of the stratospheric warming event that unfolded in December.

JMA

Shown above is a line graph showing the 30-millibar level temperature from September 2017 to the present day. I had highlighted how a stratospheric warming event appeared imminent in late November, though as mentioned this was not expected to be a sudden stratospheric warming (SSW). This is what transpired, though this warming event was prolonged and came in a number of waves after the initial failure of the first wave of warmth to penetrate the stratospheric polar vortex in the Arctic Circle. As the above graph shows, however, the warmth did eventually make its way into the stratosphere, weakening the stratospheric polar vortex and delivering the colder than normal temperatures to the eastern 2/3rds of the country to kick off the new year, as pictured below.

ESRL

Following the stratospheric warming event, the warmth in the stratospheric levels of the Arctic Circle has dissipated, leading to the sharp drop in the 30-millibar temperature that has reversed only recently, though still remains well below normal. Similar to how a warming event in the stratosphere can lead to cooler than normal weather in the eastern 2/3rds of the country roughly 2-3 weeks later, a rapid cooling of the stratospheric Arctic Circle region can encourage warmer than normal temperatures in the United States, as the colder than normal air becomes "locked up" in the upper latitudes. We'll get to the forecasts for the medium-range shortly to see if this does end up being the case.

For the time being, though, the Pacific is showing some very interesting features that I'd like to review.

CPC

Above is a four-panel graphic showing the global orientation of 200-millibar wind speeds (color fill) and geopotential heights (contours) in five-day periods, from December 18th to the present day. From the five-day period including Christmas Day to today, the Pacific jet stream has strengthened and become extended to the waters due south of the Aleutian Islands. A rule of thumb when observing the Pacific is that an extended jet stream can portray the threat of a storm system in the following days, and this does appear to be the case for this week, but will require a post of its own. For now, it is worth noting the extended nature of the Pacific jet stream, as this will more than likely lead to an opportunity for unsettled weather in the U.S. in the next two weeks.

Let's now take a look at one of my favorite longer-range graphics to watch, the 8-10 day geopotential height anomaly forecast from the three primary global weather models.

PSU

The ECMWF model's forecast for 8-10 day geopotential height anomalies is shown on the left-most panel, with the GFS model's guidance in the middle panel. The CMC (Canada's global weather model) provides its outlook in the right-most panel.

The three models certainly have some key differences in the projected pattern for the eight to ten day period, but there are a few similarities to be gleaned from the guidance. For one, all three models indicate the presence of a Rex Block in western Canada into Alaska and the Gulf of Alaska, albeit to varying magnitudes. The GFS is most pronounced with this pattern, producing a 'textbook' Rex Block in the form of the strong, cut-off ridge over a portion of the Arctic Circle and Alaska, as well as the negative anomalies in the Gulf of Alaska, offshore the Pacific Northwest. One could argue the actual Rex Block is formed with that ridge and the apparent upper level low over north-central Canada in the GFS panel, but such an orientation (slanting to the southeast, from the ridge to the upper level low) is rather unorthodox.
The ECMWF and CMC generally agree with the idea of the Rex Block, though neither model cuts off the ridge as the GFS does, instead setting up a positive-PNA-looking ridge from the American Rockies into a portion of the Arctic Circle, as one continuous ridge. Both the ECMWF and CMC still appear to support this as a Rex Block, however, with the zonal downstream flow in the United States, save for the waters just offshore of New England, where a suppressed ridge holds in place as a consequence of strong negative geopotential height anomalies in Europe and a mixed North Atlantic Oscillation (NAO) pattern. Those two features will combine to discourage colder than normal air buckling south into the eastern two-thirds of the country, instead supporting seasonal temperatures.

It's not enough to simply eyeball weather models, however. We now turn our attention to the Madden-Julian Oscillation.

CPC

Above is the ECMWF model forecast for the Madden-Julian Oscillation (MJO), with the ECMWF ensembles displayed individually via the thin yellow lines. Although this forecast period began on January 1st and ends January 15th, more-recently issued forecasts still support the outlook of the MJO transitioning from Phase 2 to 3, and perhaps then to Phase 4, albeit at a weaker grade than the previous phases. As the screenshot below and on the right shows, from the Climate Prediction Center,

temperatures in the United States tend to be cooler than normal for much of the Central U.S. and the majority of the East U.S. during Phase 2 in the winter months. This follows the rule of thumb (portrayed by these same composites) that Phases 8, 1 and 2 of the MJO are most favorable for colder than normal and snowier than normal conditions in the eastern two-thirds of the country during the winter months. Similarly, Phases 4, 5 and 6 are most encouraging for warmer than normal temperatures in the United States for the winter months.

The expected movement into Phases 3 and 4 of the MJO by the middle of the month suggests warmer than normal temperatures for much of the United States around the same time period. This seems to go hand in hand with the three primary models' projections of the 8-10 day period, and indicates that the middle of the month (likely a little bit beyond the middle of the month) should be warmer than normal for much of the country. I expect the warmer than normal tendency to continue through the later part of January in part due to that rapid cooling in the stratosphere that was observed at the end of December, and will translate to the warmer than normal weather for the middle and end of the month.

But what should we be on the lookout for at the end of the month? Will the warmth continue, or will the cold return?

FU-Berlin

Just as this post began with the stratosphere, it will close out with the stratosphere. Above is a multi-panel forecast from the ECMWF model for different variables in the stratosphere. We will focus in on the top three panels to make some prognostications for the end of January and into early February.

The top panel shows observed zonal wind speeds at the 1-millibar level of the stratosphere - in other words, the strength of the far-upper stratospheric polar vortex. The 1-millibar level isn't as important as the 30-millibar or 50-millibar levels. As an example of this, it has not been uncommon for a warming event to occur at the 1-millibar level but not the lower levels of the stratosphere, resulting in the effects of that 1-millibar level not showing up in the troposphere. Regardless, it is useful as an early indicator of a weakening of the stratospheric polar vortex, which can then show up in weakening at other levels of the stratosphere. The black line shows the ECMWF forecast of the 1-millibar wind speed, and as the wind speeds strengthened with the rapid cooling of the stratosphere (indicating a strengthening of the stratospheric polar vortex, as is to be expected), they look to again weaken over the next couple of weeks, albeit still remaining well above levels that would imply a "weak" stratospheric polar vortex. If you're a 'glass half-full' person and enjoy cold weather, then this is a good sign. Personally, the weakening of the wind speeds from a projected 90 m/s to a projected 50 m/s at the end of the forecast period, while a notable change, isn't something to really pay attention to just yet. If the wind speeds continue to weaken, then it could be something to take note of, but for now the upper-stratospheric polar vortex looks set to remain in firm control.

A similar forecast is shown in the second panel for the 10-millibar zonal wind speed (blue line) and the 30-millibar zonal wind speed (red line), where a strengthening of the wind speeds is then followed by a weakening, though still to rather-elevated speeds. To make matters worse for those who are seeking some colder weather, both the geopotential flux and heat flux indicators in the third panel are set to significantly increase during the forecast period, usually a positive sign for those who want the polar vortex to weaken. However, if you've analyzed these graphs before, you'll note how the EP-flux on the bottom panel is pointing up and to the left, when looking at it two-dimensionally. Stratospheric warming events are most probable when the geopotential and heat fluxes are high (which is forecasted) and the EP-flux arrows are pointed directly up (which is not forecasted). As such, the stratospheric polar vortex looks set to keep the colder air locked up in the Arctic Circle through at least the second-to-last week of January, helping to severely hurt any chances of a sustained colder-than-normal pattern for the eastern two-thirds of the country through at least early February.

To Summarize:

- The recently-discussed stratospheric warming event has ended and has been followed by significant cooling.
- Atmospheric oscillations, the stratosphere, and medium-term weather model guidance agree on warmer than normal temperatures more likely than colder than normal temperatures in the eastern two-thirds of the country through at least mid-January.
- The stratosphere supports this likelihood of a warmer than normal weather pattern persisting into at least early February.
- A separate post on the potential for a strong storm system this week is forthcoming.

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